Conventional lasers frequently incorporate a light energy monitor proximate to the laser for monitoring the intensity of the laser light. One such system is described in U.S. Pat. No. 5,450,207,entitled "Method and Apparatus for Calibrating a Laser Wavelength Control Mechanism" by Igor Formenkov, assigned to the present assignee and incorporated by reference. Typically, the light energy monitor is a photodetector which produces an output current proportional to the light intensity impinging on the photodetector.
Lasers are frequently used in semiconductor wafer fabrication systems to selectively expose a layer of photoresist on the wafer. A mask (or reticle) is interposed between the laser light and the wafer to allow the selective exposure. State of the art exposure systems step or scan the light exposure pattern over the wafer surface and include a relatively complex optical system to direct the laser radiation onto the wafer surface.
The amount of light required to adequately expose the photoresist during a step or scan is a product of the light intensity and the length of time that the light impinges upon the wafer. Accordingly, the intensity of light must be known accurately in order to determine the required duration of the light impinging on the wafer. Typically, the laser light is generated in pulses, and the number of pulses needed to adequately expose the photoresist is determined based upon the intensity of the laser light. Due to variations in the pulse energy, the number of pulses in a single exposure is usually large (e.g., greater than 20) so that variations in individual pulses do not significantly affect the total exposure energy.
Applicants have discovered that the stepper or scanner optics give rise to fairly unpredictable attenuation of the laser light intensity impinging upon the wafer. Since the light intensity detector is located near the laser output, there is no way to take into account any unpredictable light attenuation by the stepper or scanner optics. Accordingly, the actual light intensity impinging upon the wafer is not known, and the wafer processing system is not optimized.
The above-described problem is more significant when a large excimer laser system is used in conjunction with a stepper or scanner, since the laser system is typically physically spaced from the stepper or scanner by a number of meters and requires additional beam steering optics.